1. Field of the Invention
The invention lies in the field of light-emitting transmitting devices, in particular for purposes of optical data transmission, which are equipped with a so-called monitor unit for the purpose of power control. Specifically, it is desirable for many applications for the output power of a transmitter, which are usually lasers, to be kept as constant as possible--independently, for example, of temperature fluctuations and aging effects. In conventional edge-emitting or side-emitting lasers it is comparatively unproblematic to have a lateral arrangement of a monitor unit, because the monitor unit can be arranged to face the laser side or edge not provided for outputting useful radiation. The (lost) radiation emerging on this side is thus used for control purposes.
Recent developments and progress in production engineering have increasingly rendered possible the economic production of so-called vertically emitting transmitters (VCSEL=vertical cavity surface emitting laser). These transmitters, however, likewise require a control unit with a monitoring device if there is a high demand placed on the quality of a constant laser output power.
With regard to this problem, the introductory text of the international publication WO 95/18479 describes a basic design in which a fraction of the radiation emitted by a transmitter (VCSEL) with a vertically emitting emission zone is directed via a beam splitter onto a monitor unit in the form of a photodetector (photodiode). The output signal of the photodetector is applied to a control circuit which, in accordance with the photodetector output signal, influences the drive current of the laser in such a way that a desired transmitting output power is achieved. That design is relatively complex.
Against that background, WO 95/18479 describes an optical transmitting device in which a vertically emitting laser (VCSEL) has an integrated photodetector which evaluates radiation emerging at the underside of the laser. That design is also comparatively complicated and assumes an integral design of transmitter and monitor unit. It is therefore not possible for vertically emitting optical transmitters to be fitted subsequently or optionally with a monitor unit.
A further transmitting device is known from U.S. Pat. No. 5,809,050 (European patent application EP 0 786 838 A2). In that prior art transmitting device (FIG. 4E), provision is made for the purpose of implementing an integrated controlled laser light source of a vertically emitting laser (VCSEL) as an optical transmitter which has only a single emission zone emitting light vertically. Also provided is a monitor unit with an optically sensitive reception zone for controlling the power of the optical transmitter. A beam splitter which on the transmitter side splits up the emitted radiation into a fraction and a remaining part, is arranged between a coupling zone, which can be formed, for example, by an optical conductor connector receptacle, on the one hand, and the emission zone and reception zone on the other hand. The remaining part is output as useful radiation toward the coupling zone, while the fraction passes to the reception zone.